You can buy it in the garden centre - I don't really know of any recognised brand names. Using a mix of top soil and compost will help with fertility. You can also get topsoil on Freecycle from people having building work done - it can be a bit more of a risk, however, as you don't know what's been growing in it!

I bought a mixture of manure, organic compost, and cheap grow bags. The best deals, in multi purchases, were from independent nurseries, but it is still quite an expensive way of doing things. I intend to also top dress with organic compost, as time goes by.I'd also be wary of top soil from an unknown source. You could be introducing all sorts of weeds.